Insulated Raft Foundation

A fully insulated raft foundation can be a very efficient and environmentally friendly solution because it uses less concrete and entails less digging and spoil removal than conventional foundation systems. The main limitation to this kind of foundation is the soil type that the site is situated on. Where there is shrinkable clay in the ground, as is quite commonplace,  there is a possibility of heave (expansion  or shrinkage of the clay), and so the soil may not have sufficient bearing capacity for this method to be used.

Before any groundwork commencement, a site survey should be  carried out to ascertain the soil type enable a calculation of the ground bearing capacity. If there is any risk of soil contamination this can be checked at the same time. If the soil is suitable the soil is scraped off to a depth of around 600mm, a layer of aggregate compacted across the whole area and carefully levelled. The pre-formed polystyrene (EPS) formers are then installed around the perimeter and in-filled with EPS sheets to a thickness of 300mm. If any under floor heating is required this is laid on top and reinforcing steel laid on top. The 100mm concrete is then laid on top, this is power floated  to a smooth surface as this is your finished floor which can be directly carpeted / have timber flooring laid on top, or can even be polished to a mirror finish though this can be expensive.

The main four processes involved are illustrated below:

found 01 found 02 found 03 found 04
The system consists of clearing the top soil to 600mm below finished floor level, then covering with compacted hardcore. On top of this expanded polystyrene blocks are laid around the perimeter, followed by polystyrene sheets inside the building. On top of the polystyrene, steel mesh is installed along with under floor heating pipes if required. Once everything is in place the slab can be poured which forms your finished floor screed.

The passivhaus version provides a thermally bridge free foundation, or the less costly version a very efficient solution.

The drawings below shows the thermally bridge free passivhaus foundation version which has the outside of the timber sole plate supported by a thermally separated ring beam to the main slab so there is minimal thermal bridging through the sole plate / slab interface. Alternatively, a solid sole plate can be used which can overhang the polystyrene for a passivhaus solution:

edge detail olof no labelsl 2015 02 05soleplate

Slab advantages:

    • Concrete in foundation is all warm side of the insulation
    • Quick to install once the site is prepared
    • Simple / elegant
    • Various options varying in price, down to passivhaus thermally bridge free options
    • Specialist process
    • Surface power float finished ready to be tiled / carpeted – no screed


    • Expensive, particularly the passivhaus version
    • Not suitable for certain ground types – shrinkable clay for example